1. Field of the Invention
This invention relates to integrated circuit packaging technology, and more particularly, to a TBGA (Tape Ball Grid Array) package assembly with grounded heat sink and method of fabricating the same.
2. Description of Related Art
BGA (Ball Grid Array) is an advanced type of integrated circuit packaging technology which is characterized by the use of a substrate having a front surface for mounting a semiconductor chip and a back surface provided with a ball grid array (i.e., an array of solder balls) to serve as external connecting points for the packaged semiconductor chip to be electrically coupled to external circuitry such as a printed circuit board (PCB).
TBGA (Tape Ball Grid Array) is an improved type of BGA technology which is characterized by the use a of thin tape, typically made of polyimide, as the substrate for mounting the semiconductor chip and the ball grid array. The TBGA technology allows the overall package size to be made very compact in size.
FIG. 1A is a schematic sectional diagram showing an example of a conventional TBGA package assembly. As shown, this TBGA package assembly includes: (a) a tape 110; (b) a heat sink 120 having a downward-facing chip-mounting cavity 121 and adhered to the tape 110 by means of an adhesive layer 113; (c) a semiconductor chip 130 mounted in the chip-mounting cavity 121 of the heat sink 120; (d) an array of solder-ball pads 140 provided on the back surface of the tape 110, including a subgroup of grounding solder-ball pads 141 and a subgroup of I/O solder-ball pads 142; (e) a solder mask 150 formed on the back surface of the tape 110 while exposing the solder-ball pads 140; (f) a plurality of grounding plugs 160 formed in the tape 110, which connect the heat sink 120 to the grounding solder-ball pads 141; (g) a set of bonding wires 170 bonded to the semiconductor chip 130, including a subset of grounding wires 171 and a subset of I/O wires 172; (h) a ball grid array 180 (i.e., an array of solder balls) attached to the array of solder-ball pads 140, including a subgroup of grounding solder balls 181 and a subgroup of I/O solder balls 182; (i) an encapsulation body 190 for encapsulating the semiconductor chip 130.
The foregoing package assembly is characterized by that the semiconductor chip 130 can be connected to external grounding points (not shown) via a grounding path composed of the grounding wire 171, the heat sink 120, the grounding plugs 160, the grounding solder-ball pads 141, and the grounding solder balls 181, as indicated by the dotted arrows in FIG. 1A. During SMT (Surface Mount Technology) process for mounting the finished TBGA package on a PCB (not shown), it allows the packaged semiconductor chip 130 to be connected to the PCB""s grounding lines (not shown) via the grounding solder balls 181.
Further, as shown in FIG. 1B and FIG. 1C, by conventional TBGA technology, each grounding plug 160 is formed by first punching a hole 161 through each grounding solder-ball pad 141 into the tape 110, and then filling each punched hole 161 with an electrically-conductive material, such as solder or silver epoxy.
The forgoing process step for forming the grounding plugs 160, however, has the following drawbacks.
First, as illustrated in FIG. 1C, since the polyimide-made tape 110 is non-wettable to solder, a tiny gap 162 would be undesirably left between each resulted grounding plug 160 and the inner wall of the punched hole 161, making the grounding plug 160 only bonded to the heat sink 120 but unbonded to the inner wall of the punched hole 161 and thus loosely secured in position. In addition, since the punched hole 161 is formed by punching through each grounding solder-ball pad 141, it will reduce the effective solder wetting area of the grounding solder-ball pad 141, making the substantially attached grounding solder ball 181 to be reduced in overall bonding strength, resulting in a weak ball shear strength to the grounding solder ball 181. This would undesirably cause potential reliability problem to the finished TBGA package.
Second, since the opening of the punched hole 161 for the grounding plug 160 is on the back surface of the tape 110, it requires the solder used to form the grounding plug 160 to be filled from the bottom side of the package assembly into the punched hole 161, which is considered a difficult process step.
Third, it would undesirably cause some moisture or air to be trapped in the punched hole 161, undesirably making the resulted grounding plug 160 to be poorly bonded to the heat sink 120 and bulged out in position that would cause the subsequently attached grounding solder balls 181 to be also bulged out in position with respect to the I/O solder balls 182, thus making the overall ball grid array 180 uncoplanarized that would degrade the quality of SMT (Surface Mount Technology) process for mounting the finished TBGA package on a printed circuit board (not shown).
Patents related to TBGA fabrication, include, for example, the U.S. Pat. No. 6,020,637 entitled xe2x80x9cBALL GRID ARRAY SEMICONDUCTOR PACKAGExe2x80x9d. By this patented technology, however, since it also involves the fabrication of grounding plugs by punching through the grounding solder-ball pad and the tape, the above-mentioned drawbacks still exist.
It is therefore an objective of this invention to provide a new TBGA technology which can allow grounding solder balls to have greater ball shear strength so as to make them more firmly secured in position.
It is another objective of this invention to provide a new TBGA technology which allows the electrically-conductive material used to form the grounding plug to be filled from the top side of the package assembly instead of from the bottom side, so as to make the filling step easier to carried out.
In accordance with the foregoing and other objectives, the invention proposes a new TBGA package and method of fabricating the same.
The TBGA technology according to the invention is characterized by that the grounding plug is formed by first forming a via hole in the heat sink and a via hole in the tape without penetrating through the grounding solder-ball pad, and then filling an electrically-conductive material, such as solder or silver paste, into the heat-sink via hole from the top of the package assembly until filling up the tape via hole and the heat-sink via hole. As the semiconductor chip is mounted in position, its grounding pads are electrically bonded to the heat sink, thereby allowing the semiconductor chip to be externally grounded through the grounding plug, the grounding solder-ball pad, and the grounding solder ball.
The TBGA technology of the invention allows the resulted grounding plug to be firmly secured in position due to the filled solder being wettable to the heat sink, thereby providing a greater ball shear strength to the grounding solder ball that is subsequently bonded to the grounding plug. The finished TBGA package is therefore more assured in the reliability of its grounding structure.